Author: Jim Beauchamp, Regional Power Specialist, Avnet EM
Board mounted power modules come in a wide variety of input/output voltage configurations, power levels and package types. They typically incorporate various electronic components including power semiconductors and passives. The non-ideal characteristics of these components result in power loss in the form of heat dissipation. Since PCB real estate is at a premium in most end products, there is a necessity to produce greater amounts of power within a given area. Hence, the proper selection of a power module requires careful review of several specified parameters including thermal performance. To obtain maximum power and maintain reliable operation, the thermal derating of a given module must be correctly calculated and demonstrated.
Power module thermal derating is defined as the difference between the specified output power and the recommend output power for given thermal environment. It is often represented graphically as ambient temperature versus output current (or power) within a module data sheet or application note. Derating curves provide a method to estimate the maximum recommended output power at a prescribed temperature and air flow, in order to achieve the specified failure rates.
For a given output current, as the ambient temperature rises, the component temperature within the module will also rise. Eventually, reaching a critical limit at which one of the components will degrade and the component’s mean time between failures (MTBF) is reduced. It is at this ambient temperature, with addition of some margin, that the module derating is specified. Hence, if the ambient temperature is decreased, there is an allowance for more thermal losses, enabling an increase in output current. Also, by increasing the airflow, convective heat loss is increased and a similar allowance can be created. Ultimately, the current is limited at a finite value (i.e. 25A) due to a current limit that is implemented to protect components within the module that can be damage by excessive current flow.
Figure 1 shows a typical derating curve for a 25A module.
Note that in this example the air flow is transverse, which indicates that the module is oriented with the longest dimension in the direct path of the air flow. LFM is defined as Linear Feet per Minute of air flow. Natural convection is defined as no air flow other than the air flow generated by the thermal currents created by heat generating components.
Figure 1: Typical Derating Curve for a 25A Module
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